1. Field of the Invention
The present invention relates to coal-to-liquid technology, and specifically to a system and method for liquefying coal using solvents that hydrogenate under mild conditions.
2. Description of the Related Art
Coal-to-liquid technology refers to chemical processes that convert solid coal into liquid fuels and chemicals. The hydrogen to carbon ratio (H/C, molar) of coal is about 0.8 while that of liquid fuels is about 2.0. The main functions of the coal-to-liquid processes are breakage of the coal's molecular size and addition of hydrogen into coal, or in other words, destructive hydrogenation of coal. These processes are generally termed as coal liquefaction.
Coal liquefaction may occur by two different pathways: indirect liquefaction and direct liquefaction. The indirect method converts coal to hydrogen and carbon monoxide, and syngas by reacting coal with steam at high temperatures in an oxygen-starved combustion process. Direct liquefaction includes reaction of coal with hydrogen in a manner that coal becomes liquid. However, direct coal liquefaction has been historically carried out with hydrogen gas, which requires high temperature and pressure. In an example, direct coal liquefaction may involve temperatures in excess of 450° C. and 2000 psig pressure.
Tetralin has been used as a donor solvent. However, a large overpressure of hydrogen and high temperature is needed to transfer the hydrogen from the gas phase to naphthalene, which is produced when tetralin is dehydrogenated as it transfers hydrogen to coal molecules. Thus, in situ re-hydrogenation during liquefaction can be rather costly.
In view of the limitations discussed above, there exists a need for a method of coal liquefaction utilizing an inexpensively produced, effective hydrogen donor solvent to digest coal.